1. Field of the Invention
The present invention relates to an image reading apparatus for reading the images of originals and to an image forming apparatus having the image reading apparatus.
2. Related Background Art
Conventionally, there has been available an image forming apparatus such as a digital copying machine which is equipped with an original feeder (hereinafter referred to as ADF (auto document feeder)) such as a duplex auto original feeder. In a digital copying machine equipped with an ADF, when an original in the ADF is to be copied, a reading apparatus provided below the ADF is moved to a set position, i.e., a position directly below a platen roller in the ADF, by a motor, and the apparatus is held at the position. Then light is emitted from a lamp unit, the original being transported between the platen roller and a platen glass plate to be thereby scanned, and the reflected light from the original being detected by the reading apparatus. Thereafter, processing is performed in a printer portion on the basis of the detected data, thereby obtaining a copied image.
As shown in FIGS. 14A, 14B, and 14C, an image reading portion of an ADF is composed of a platen roller 224, an upstream side transport roller 222, and a delivery side transport roller 223. When transporting an original, it is necessary to transport the original between the platen glass plate and the platen roller 224 at a constant speed and along the platen glass plate.
However, as can be seen from FIG. 14A, as the reading position is deviated from the position directly below the platen roller 224, the gap between the platen glass plate and the platen roller 224 is enlarged, and the original is raised from the platen glass plate under the influence of the rollers and guides on the upstream and downstream sides, resulting in unintended generation of a blurred image, or a fogged image, blank image, etc. due to variation in light quantity.
Further, as shown in FIG. 14B, at the instant that the trailing edge of the original leaves the guide, the original attitude is changed from the round state to the horizontal state, resulting in unwanted generation of a blurred image, or a fogged image, blank image, etc. due to variation in light quantity.
Further, as shown in FIG. 14C, before and after the leading edge of the original passes a jump stand for guiding the original to the delivery side transport roller 223, the original attitude is changed, resulting in undesired generation of a blurred image, or a fogged image, blank image, etc. due to variation in light quantity.
Further, in the ADF, it can happen that toner, ink or the like adhering to the surface of the original being transported is separated therefrom, and accumulated on the platen glass plate. When the dust thus accumulated exists directly above the original reading position, the dust may be taken for an image, with the result that it is erroneously recognized and read as a streaked image.
Since the gap between the platen glass plate and the original is kept minimum by the platen roller 224, the accumulated dust can be automatically removed by being rubbed away by the original being transported (self-cleaning effect). The nearer the position directly below the platen roller, the more remarkable becomes this self-cleaning effect. Thus, it is necessary to set the reading position directly below the platen roller. However, this is hard to realize due to insufficiency in the precision in parts and the precision in the original reading position, and a streaked image as mentioned above can be generated in the image while it is read at a position away from the platen roller 224.